Why it works
Planning meetings run long when the backlog isn't ready — the team spends the session writing tickets instead of committing to them. A standing reminder the day before makes prep a habit, so planning starts with a groomed list. The checklist is the same every sprint, which is exactly why a frozen recurring email fits: set it once and it lands before every planning, forever.
Make it yours
Match the cadence to your sprint length (weekly, every two weeks, monthly). If a specific person owns grooming, address it to them; if it's shared, send it to the whole team and let the "reply if something needs a conversation" line surface the risky items.